1. Field of the Invention
The present invention relates to a photovoltaic device, a photovoltaic module, and a method of establishing a photovoltaic system.
2. Description of the Prior Art
A photovoltaic module that is intended to reduce the area of a photo detector by increasing a ratio of the area of a light-incident plane of a concentrator to the area of the photo detector (area of light-incident plane/area of photo detector), that is, a concentrating ratio, is disclosed, for example, in Japanese Patent Laid-open (International Publication) No. Hei 6-511602.
The cross-section of the above-mentioned photovoltaic module is shown in FIG. 3(a). A first concentrator 16 and a second concentrator 17, each of which is composed of a light-incident plane 1, a reflection plane 2, and a medium 4, are arranged laterally, symmetrically to each other. An angle formed between the light-incident plane 1 and the reflection plane 2 is made smaller than 45.degree., to make the concentrating ratio larger than 1, thereby making the area of a photo detector 5 smaller than that of the light-incident plane 1. The concentrating ratio can be made larger by making smaller the above angle; however, since the theoretical limit of the concentrating ratio lies at about 10, the lower limit of the above angle is set at about 5.7.degree..
A photovoltaic module including a cover glass portion 200 provided on first and second concentrators 16 and 17, shown in FIG. 4(a), is disclosed in a document entitled "Prototype Photovoltaic Roof Tiles, 13th European Photovoltaic Solar Energy Conference, Nice, France, October 1995, pp. 1483-1486".
This document also discloses a photovoltaic module as shown in FIG. 5(a), in which the light-receiving plane of a photo detector 5 is arranged in the direction perpendicular to the light-incident planes of the first and second concentrators 16 and 17.
The above-described prior art photovoltaic modules are each established such that the light-incident planes are tilted from the horizontal plane by an angle equivalent to the latitude of an establishing location in order to allow sunlight at the equinoxes to be made perpendicularly incident on the light-incident planes. For example, if the module is established at a location in Tokyo which lies at 34.degree. in the northern hemisphere, the light-incident planes are tiled to the south by 34.degree. from the horizontal plane.
For the module with no glass cover portion 200, as shown in FIG. 6, the first and second concentrators 16 and 17 are generally designed to trap sunlight incident from the narrow portion (right end) in cross-section of, for example, the concentrator 17 in the direction nearly parallel to the light-incident plane (incident angle: nearly 90.degree.); sunlight incident at an angle which becomes small from nearly 90.degree. to 0.degree. (normal to the light-incident plane); and sunlight incident at an angle which becomes large from 0.degree. to 24.degree.. These states are illustrated on the upper sides of the first and second concentrators 16 and 17 in FIG. 6. It should be noted that the incident angle 24.degree. corresponds to the tilt angle of the earth axis with respect to the revolution plane of the earth. The moving angles of sunlight throughout the year are illustrated on the upper side in FIG. 6.
If the photovoltaic module is established such that sunlight 400,401 at the spring equinox and sunlight 400,401 at the autumnal equinox are both made perpendicularly incident on the light-incident planes 1 as shown in FIG. 3(a), the module can effectively generate power throughout the year including a period from the spring equinox to the summer solstice 24.degree. offset from the spring equinox and a period from the autumnal equinox to the winter solstice 24.degree. offset from the autumnal equinox.
The photovoltaic module, however, is not necessarily established such that sunlight at the equinoxes is made usually perpendicularly incident on the light-incident planes. That is to say, the module is sometimes established in contact with a curved plane of, for example, a roof. In this case, as shown in FIG. 3(b), the sunlight 400,401 at the spring equinox and sunlight 400, 401 at the autumnal equinox are both made obliquely incident on the light-incident planes 1. As a result, the sunlight 400 is trapped by the second concentrator 17; however, the sunlight 401 is not trapped by the first concentrator 16. This cause an inconvenience in reducing the light trapping efficiency of the module.
For the module with the glass cover portion 200 as shown in FIG. 4(a), the plane causing total reflection, that is, the light-incident plane is shifted from a lower plane 32 of the cover glass sheet 200, which is equivalent to the light-incident planes of the module shown in FIG. 3(a), to an upper plane 1 of the cover glass portion 200. As a result, of the sunlight allowed to be trapped for the module with no cover glass portion 200, light 402 cannot be trapped by the module with the cover glass portion 200 and thus escapes to the outside. This causes an inconvenience in reducing the light trapping efficiency of the module. The above reduction in light trapping efficiency similarly occurs in the case where the module with the cover glass portion 200 is, as shown in FIG. 4(b), established such that sunlight at the equinoxes is not made perpendicularly incident on the light-incident plane. To be more specific, as shown in FIGS. 4(a) and 4(b), the light 402 having been made incident on the second concentrator 17 is reflected from the reflection plane 2, being subjected to total reflection from the light-incident plane 1, and enters the first concentrator 16. At this time, since the first concentrator 16 is arranged laterally symmetrically to the second concentrator 17, the incident light 402 entering the first concentrator 16 cannot be trapped in the photo detector 5 and thus escapes to the outside.
Even for the module shown in FIG. 5(a) and the module, shown in FIG. 5(b), established such that sunlight at equinoxes is not made perpendicularly incident on the light-incident planes, since first and second concentrators 16 and 17 are arranged in the same manner as that for those shown in FIGS. 3(a) and 3(b) respectively, these modules have the same problem associated with the reduction in the light trapping efficiency.